Wikipedia:Reference desk/Archives/Mathematics/2016 August 25

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August 25

Looking for 2D conic section graphing utility

I currently use DESMOS, which I love in general, but it doesn't do anything with foci and directrix lines. I'd like it to show those, say if I give it the equation of a parabola, in any form, or I'd like to be able to provide 2 of 3 of the focus, vertex, and directrix and have it plot the parabola and give me the equation, in whatever form I select. I'd also like similar capabilities with an ellipse or hyperbola. Looking for something free. Thanks, StuRat (talk) 16:33, 25 August 2016 (UTC)

Have you tried GeoGebra? Sławomir Biały (talk) 18:23, 25 August 2016 (UTC)

Nope. Does it do the type of things I'm looking for ? StuRat (talk) 03:30, 26 August 2016 (UTC)

Try it and see. It has probably the best support for conic sections you're likely to find anywhere. And it's free. Sławomir Biały (talk) 10:07, 26 August 2016 (UTC)

Thanks. This applet seems to do half of what I want for parabolas (find the equation given the focus, vertex, and directrix), although it doesn't give me a choice of equation forms: [1]. There are similar apps for creating an ellipse or hyperbola and reporting the equation. One shortcoming is that these graphs only go between -20 and +20 in the x direction and -9 to +9 in the y direction, and don't appear to be zoomable. Also, I need to be able to do the reverse, and enter the equations to find all those control points and lines. StuRat (talk) 17:21, 26 August 2016 (UTC)

I don't know what you mean. Geogebra is a fully featured computer algebra system. Download it onto your computer and run that computer algebra system. It definitely supports zooming. Sławomir Biały (talk) 17:58, 26 August 2016 (UTC)

I was looking at users' applets built on GeoGebra, not the full application, hoping to find something "light" that does just what I need. How's the learning curve with the full application ? StuRat (talk) 20:11, 26 August 2016 (UTC)

May the force be with you

In this calendar for August 2016

Su Mo Tu We Th Fr Sa
..  1  2  3  4 .. ..
..  8  9 10 11 .. ..
.. 15 16 17 18 .. ..
.. 22 23 24 25 .. ..
.. .. .. ..  

you choose a number then cross out all other numbers in the same column and row. Rinse and repeat three times so that you are left with four numbers, and however you do it they will always sum to 52. This is called a "force", and you can do it with any block of sixteen dates in any month, although the magic sum will be different if the numbers in the block are different.

I know of a party trick in which the performer tells a guest he can read his mind. He asks the guest to choose any two - digit number then tells him to perform certain arithmetical operations. When he has finished, he writes a number on a piece of paper and asks the guest for the result of the calculation. When the guest answers, the performer holds up the paper to show that the number is the same (assuming that the guest has done the calculations correctly). Is this also known as a "force"? 213.107.114.104 (talk) 16:55, 25 August 2016 (UTC)

In magic, "force" is usually used for a trick of Equivocation, that is one where the magician gives the illusion of choice, but really makes the choice for the mark. This trick is simply "math"... --Jayron32 17:41, 25 August 2016 (UTC)

I would call it obfuscation, since the trick works by creating several intermediate steps between the guest's choice of numbers and the final answer, in order to hide that the answer is just a function of the input. OldTimeNESter (talk) 16:25, 26 August 2016 (UTC)

It sounds to me like the answer is constant, not a function of the input at all. For an extremely simple example, you could have them multiply their number by 0 and then show that you had the answer, 0, written down on the paper before they even started. Obviously that wouldn't fool anyone, but you could make a far more complex series of math steps that ends up doing the same thing. To make the trick a bit better, I'd suggest that instead of one answer, you make it so there are a small number of possible answers, say 10. You then ask them for their answer, and pull a slip of paper out of whichever pocket has that number. StuRat (talk) 17:46, 26 August 2016 (UTC)

1089 (number)#In magic is a famous example but it starts with a 3-digit number. PrimeHunter (talk) 18:17, 26 August 2016 (UTC)